This invention concerns the analysis of image data to detect monochrome images.
The automatic detection of image characteristics is desirable for many applications including the classification of images in a library and the control and monitoring of broadcasting and similar installations. A useful image characteristic is whether or not it contains a representation of the colours of objects.
Systems which process colour images generally define pixels of the image by three parameters. The two most commonly used parameter sets are: Red Green and Blue; and, Luminance Red colour difference and Blue colour difference. (Usually the colour difference signals are proportional to the difference between the relevant colour component value and the luminance value, other colour difference signals are known.) Conversion between these two systems is possible by linear transformation.
It is known to recognise images lacking colour information by looking for zero, or low, values of the colour difference parameters. However this method is unreliable for images lacking strongly coloured content, such as snow scenes. This method will also not distinguish monochrome images which have been given an overall colour wash (e.g. a sepia tone), from images in which the colour content describes the objects in the image. There are other sources of “spurious” colour information such as the “cross-colour” artefact of composite colour coding systems (PAL, SECAM etc.), and noise from analogue transmission and storage techniques.